In a wire bonding process, electrically conductive wires are bonded between electrical bonding pads found on semiconductor devices, such as between a semiconductor die and a substrate onto which the die is attached. The substrate is usually a semiconductor leadframe. The electrical connection could also be made between bonding pads found on separate semiconductor dice. The bond is formed by a bonding tool which may be in the form of a capillary attached to an ultrasonic transducer for generating ultrasonic energy to the capillary tip.
In modern day wire bonders for making so-called “ball-bonds”, a bondhead which carries the bonding tool is designed to execute a rocking motion about a suitably located pivot. For ultrasonic bonding, the bonding tool is an ultrasonic transducer mounted onto the bondhead, the ultrasonic transducer comprising a piezoelectric driver stack coupled to a horn, and a capillary at an end of the horn. Bonding wire, which is typically made of gold, aluminum or copper, is fed from a spool of bonding wire through a hole in the capillary to the tip of the capillary. Bonding is done by welding the wire at the tip of the capillary to the bonding pad through the application of ultrasonic energy to the capillary tip.
It is common to utilize a wire clamp to control feeding of bonding wire to the capillary tip. For example, the clamp may be closed to hold onto and fix a length of wire relative to the capillary, or opened to allow wire to slide through the capillary. The wire clamp is also closed to hold the wire in position during the making of wire bonds on the bonding pads. The clamp is further commonly used to facilitate looping of a length of bonding wire between electrical bonding points on the die and/or substrate, and/or to pull and break wires from bonds after the bonds have been made. The wire needs to be held firmly, fed to the bonding site and stripped off at appropriate junctures in the process. Over the years, the operational speed of wire bonding machines has increased considerably, with the result that the wire clamp and bondhead need to be actuated at high speeds while exerting controlled force on the wire being clamped without damaging the wire.
FIG. 1 is an example of a prior art bondhead 100. The bondhead 100 generally comprises a bondhead body 102, a wire clamp 104 fixed to the body 102 and a transducer 106 mounted to the bondhead 100. The transducer 106 has a capillary 108 attached to one end of the transducer, and is generally movable in tandem with the movement of the bondhead 100.
Bonding wire 110 is fed from a spool of wire (not shown), and is relayed past the jaws of the wire clamp 104 and threaded through a hole in the capillary 108. The wire clamp 104 is arranged along the path of the bonding wire so as to control feeding of the wire to the capillary 108, in particular, to the capillary tip.
The bondhead body 102 is pivoted at a pivot point 112 for turning motion, and turning movement of the bondhead body 102 about the pivot point 112 is actuated by a bondhead actuator 114. The bondhead actuator 114 may comprise a voice coil motor including a coil that is movable relative to a magnet by way of electromagnetic interaction when current flows through the coil. When actuated by the bondhead actuator 114, the body 102 and wire clamp 104 are driven to turn along a turning arc 116. Bonding wire 110 is drawn from the spool of wire towards a bonding location when the wire clamp 104 is closed, and the bondhead 100 is turned away from the spool of bonding wire. The wire clamp 104 may further be opened and the bondhead 100 turned towards the spool in order to position the wire clamp 104 to clamp and draw more bonding wire 110.
During a bonding cycle and before starting to weld the first bond, a molten ball has to be formed at a tail end of the bonding wire 110 protruding out of the capillary tip. The molten ball is later lowered onto a bonding pad to form a first ball bond. The molten ball is formed at the end of this protruding bonding wire 110 by melting the wire through electro-sparking, so a sufficient length of wire must be available at the tail end of the bonding wire 110 to do so. An electronic flame-off (“EFO”) device creates an electrical spark and melts the wire to form the molten ball.
To leave a tail of bonding wire 110 protruding from the capillary tip after completion of a bond, the bondhead 100 has to follow a variety of programmed motions. More specifically, during ball-bonding processes, the bondhead 100 needs to move up a short distance with the wire clamp 104 open after the bonding wire 110 has been welded at a second bond location to complete a wire connection. Then, the bondhead 100 stops and the wire clamp 104 is closed to clamp the bonding wire 110. After that, the bondhead 100 moves up further to a higher position. During this further upward motion, the bonding wire 110 is pulled up and broken at the second bond location, and gets ready for the start of the first bond of the next wire connection. This is called tail formation, to ensure that a predetermined length of bonding wire protrudes from the capillary tip after each wire connection is established. The consistency of the length and linearity of the protruding wire determines the repeatability of ball formation and the ball size formed.
Another feature of the prior art bondhead 100 is that it uses an air tensioner to ensure the centering of the bonding wire 110 and the molten ball with respect to the capillary tip. This is to ensure accuracy of placement of the bonded ball at the first bond. After EFO sparking, the formed ball is pulled up by the air tensioner to sit in a central position under the capillary 108. The consistency of ball centering relies on the stability of the pulling force exerted by the air tensioner. Therefore, periodic checking and cleaning of the air tensioner is required to ensure consistency of ball centering.
The existing tail-formation process has a number of drawbacks. One drawback is that it requires precise synchronization between operation of the wire clamp 104 and motion of the bondhead 100. This becomes much more difficult when the bondhead 100 moves at very high speeds and acceleration. The process is also very demanding on the stability of the bondhead structure and motion. It is difficult to produce consistently straight tails with uniform lengths when bonding wires of smaller and smaller diameters are used. Any variation in the process causes corresponding variation of the wire shape of the next bonded wire, resulting in inconsistency. Furthermore, operational stoppages can result when the bonding wire 110, especially thin bonding wire, is broken prematurely at the second bond location when the bondhead 100 moves up while the wire clamp 104 is still open. Additionally, more process time is required to form the protruding bonding wire 110 by manipulating the bondhead 100, so that bond cycle time is increased for each bonded connection.